1. Field of the Invention
The present invention relates generally to a communication apparatus and method in a high rate packet transmission mobile communication system, and in particular, to an apparatus and method for efficiently distributing energy to symbols transmitted on a packet data channel. More particularly, the present invention relates to an apparatus and method for changing the sequence of symbols on a packet data channel so that more energy is assigned to symbols (or bits) highly influential in data reception performance to thereby achieve efficient packet data transmission, when energy (or power) is variably assigned to the packet data channel in a mobile communication system for high rate packet transmission.
2. Description of the Related Art
In general, a high rate packet transmission mobile communication system provides a high rate data service via a packet data channel (PDCH) as used in 1xEVDO or 1xEVDV. The PDCH is shared among a plurality of users in time division multiplexing (TDM). A sub-packet is a transmission unit of each TDM user data and transmitted in one or more slots.
A preamble channel (or a packet data control channel: PDCCH) transmits control information about the TDM data transmitted on the PDCH at a particular time point. From the control information, the users obtain information about the destination, length, data rate, modulation, etc. of the transmitted data on the PDCH.
The control information about the packet data includes sub-packet length, MAC (Medium Access Control) ID, data rate, modulation, payload length, sub-packet ID (SPID), and ARQ (Automatic Repeat Request) channel ID. As stated before, a transmission unit of data transmitted on a PDCH is defined as a sub-packet, and the length of a sub-packet is the temporal length of TDM data transmitted on the PDCH. The sub-packet length must be notified beforehand in a system having a variable data length. A MAC ID is a user identifier and is assigned to each user in a system access state. A data rate refers to the transmission rate of data having one sub-packet length. Modulation information represents a modulation scheme by which the transmission data has been modulated, such as QPSK, 8PSK, 16QAM, and 64QAM. Payload length is the number of information bits in one sub-packet. An SPID is a sub-packet identifier used to support retransmission. An ARQ channel ID identifies a parallel transmission channel in order to support successive data transmission to one user.
Two bits are assigned to each of sub-packet length, payload length, SPID, and ARQ channel ID, and six bits are assigned to MAC ID. Data rate and modulation are determined according to the 2-bit sub-packet length, the 2-bit payload length, and the number of Walsh functions available to the PDCH. The Walsh function information is received on a different channel. Each mobile station (MS), which receives a high rate packet data service, is assigned a MAC ID at its system access and receives a PDCCH. The MS demodulates the PDCCH and determines whether the current packet is destined for the MS from the MAC ID set in the PDCCH. If the packet is destined for the MS, the MS demodulates a received PDCH using the control information acquired from the demodulated PDCCH. The data rate and modulation method of the received sub-packet can be detected from information about sub-packet length, payload length, and the number of Walsh functions available to the PDCH.
The high-rate packet transmission mobile communication system transmits packet data control information on two PDCCHs, namely, a primary PDCCH (PPDCCH) and a secondary PDCCH (SPDCCH). The PPDCCH transmits sub-packet length information in one slot all the time, and the SPDCCH transmits information about payload length, SPID, ARQ channel ID, and MAC ID in one, two, or four slots.
These PDCCHs are transmitted in code division multiplexing (CDM) with a PDCH. That is, the PPDCCH, the SPDCCH and the PDCH are transmitted using different codes assigned to them at the same time.
FIG. 1 illustrates the relation between transmission power and time for the PPDCCH, the SPDCCH, and the PDCH in the high-rate packet data transmission mobile communication system.
Referring to FIG. 1, reference numerals 101, 102 and 103 denote the PPDCCH, the SPDCCH and the PDCH, respectively. Reference numeral 113 indicates that the channels are transmitted on a slot basis. Time is defined along the horizontal axis and the energy assigned to each channel is defined along the vertical axis in FIG. 1. Reference numerals 104 to 107 denote a one slot-sub-packet transmission, a two slot-sub-packet transmission, a four slot-sub-packet transmission, and an eight slot-sub-packet transmission, respectively. The PPDCCH 101 is always transmitted in the first slot of each packet data transmission period. For the packet data transmission periods of one, two, four, and eight slots, the SPDCCH 102 is transmitted in the first one, two, four, and four slots, respectively. Reference numeral 108 denotes transmission power available to a base station (BS). Reference numerals 109 to 112 denote PDCHs transmitted in one, two, four, and eight slots. The remaining power from the overall available transmission power of the BS minus the sum of transmission powers assigned to the PPDCCH 101 and the SPDCCH 102 is available to the PDCH 103.
In the case of the PDCH 109 transmitted in one slot, the PPDCCH 101 and the SPDCCH 102 are transmitted contemporaneously over the whole PDCH transmission period. Thus, the transmission power of the BS does not change. On the contrary, in the cases of the PDCHs 110, 111 and 112 transmitted in two or more slots, the power assigned to the PDCHs changes on a slot basis during sub-packet transmission because the PPDCCH 101, the SPDCCH 102, and the PDCH 103 differ in transmission duration.
FIG. 2 is a block diagram of a forward transmitter for transmitting a forward PDCH (F-PDCH) in a conventional high-rate packet data transmission mobile communication system.
Referring to FIG. 2, an encoder 201 encodes an information bit stream of a PDCH and outputs code symbols. A scrambling code generator 202 generates a scrambling code by which the packet data is to be scrambled. A scrambler 203 scrambles the code symbols with the scrambling code. A channel interleaver 204 interleaves the scrambled symbols according to a predetermined interleaving rule. A puncturer 205 punctures the interleaver output in a predetermined puncturing pattern. A modulator 206 modulates the output of the puncturer 205. A symbol demultiplexer (DEMUX) 207 demultiplexes the modulated symbols according to the number of sub-channels. A 32-chip Walsh cover 208 spreads the output of the symbol DEMUX 207 with a predetermined Walsh code of length 32. A gain controller 209 adjusts the gain of the Walsh cover output. A Walsh chip level summer 210 sums the output of the gain controller 209 at a chip level.
The encoder 201 is a turbo encoder. Turbo-coded symbols include systematic symbols and parity symbols. In the nature of turbo coding, reception performance of systematic symbols significantly influences overall throughput relative to parity symbols. That is why when turbo coding is used for channel coding and the transmission energy of a PDCH is variable as illustrated in FIG. 1, data throughput depends on the positions of systematic symbols in a transmission period, that is, energy assigned to the systematic symbols.
FIG. 3 illustrates an exemplary disposition of systematic symbols output from the puncturer 205 illustrated in FIG. 2. Referring to FIG. 3, reference numerals 301 to 304 denote a one-slot PDCH transmission, a two-slot PDCH transmission, a four-slot PDCH transmission, and an eight-slot PDCH transmission, respectively. As stated before in connection with FIG. 1, power assigned to the PDCH varies on a slot basis. Reference numeral 305 denotes the positions of the systematic symbols in each sub-packet transmission period. As illustrated in FIG. 3, the systematic symbols are concentrated on a starting portion having the lowest power in each sub-packet transmission period.
FIG. 4 illustrates another exemplary disposition of systematic symbols output from the puncturer 205 illustrated in FIG. 2. In FIG. 4, the systematic symbol disposition occurs when the turbo-coded symbols are repeated according to a predetermined repetition factor. A sequence repeater (not shown) for repeating the sequence of code symbols can be configured between the channel interleaver 204 and the puncturer 205 in the transmitter of FIG. 2, by way of example.
Referring to FIG. 4, reference numerals 401 to 404 denote a one-slot PDCH transmission, a two-slot PDCH transmission, a four-slot PDCH transmission, and an eight-slot PDCH transmission, respectively. As stated before in connection with FIG. 1, power assigned to the PDCH varies on a slot basis. Reference numeral 405 denotes the positions of systematic symbols in each sub-packet transmission period. As illustrated in FIG. 4, the systematic symbols are distributed evenly across each sub-packet transmission period.
The above-described systematic symbol dispositions illustrated in FIGS. 3 and 4 adversely affect reception performance in the nature of turbo coding.